Composite Geopolymeric Binders of Fly Ash, Effect of the Particle Size and Addition of Calcium Aluminate Cement
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Composite Geopolymeric Binders of Fly Ash, Effect of the Particle Size and Addition of Calcium Aluminate Cement Cardona-Hernández L.L1. Escalante-García J.I.1 1 Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Unidad Saltillo. ABSTRACT An investigation on composite geopolymeric binders, based on alkali activated fly ash (PFA) substituted with low-alumina calcium aluminate cement (CAC), was carried out using a Factorial experimental design in which the factors and levels were: %Na2O, 8-12%; modulus of the solution Ms=SiO2/Na2O =0 - 2, 10-30 wt% of CAC and fineness of PFA (D90) from 161.8 to 6.46 microns. The contribution of each factor was estimated with the 28-day compressive strength as the response variable. The curing temperature was 24h@60°C, and then at 20°C until mechanical testing. The specimens were also characterized by XRD and SEM. The results showed that the grinding modified the morphology of the PFA without changing the crystallographic or chemical characteristics as detected by XRD; and improved the mechanical properties of the geopolymers. The strength increased notably with the Ms up to 1, and reduced for Ms >1; the strength increased with the %Na2O and %CAC. Electron microscopy showed a higher densification at smaller PFA particle size, and the CAC addition promoted the formation of zeolite and Na2O-Al2O3-SiO2-H2O products. INTRODUCTION Considerable research has focused over the last decade on design of inorganic polymers, due to the potential wide range of applications of geopolymers [1, 2]. Theoretically, all aluminosilicate materials can be alkali activated, but many authors agree that the raw materials must meet various conditions in order to be able to make geopolymers. The raw materials should be soluble in basic media, to favor the availability of Al2O3 and SiO2 [3] into the solutions, which would further lead to the solidification of new compounds with binding properties. PFA is a byproduct of burning granulated coal in power plants, collected before the flue gases are released the atmosphere [4]; its reactivity depends on its chemical composition, amorphous fraction and other properties, which in turn depend on the composition of the coal and the burning process [5]. F-type PFA consists mainly of SiO2, Al2O3 and Fe2O3, and C-Type contains also CaO [6]. The fineness of the PFA significantly affects its reactivity when it is used as pozzolan or as an alkali activated raw material. Type F-type with a reactive Si/Al 14) has a negative effect on the strength [9]. Calcium Aluminate Cements (CAC) are hydraulic binders that form hydrates such as CAH10 and C2AH8 at room temperature; these are metastable and inevitable evolve to the thermodynamically stable form, C3AH6 with a drastic change in density, which has limited their
extensive use in concretes for construction. This research explored the use of calcium aluminate cement (CAC) as a source of reactive Al towards the formation of geopolymers based on PFA. The formation of the reaction products of CAC can also co
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